A power module is used to selectively deliver power to a load. The primary function of a power module is provided by a number of switching semiconductor devices (e.g., transistors and diodes) within the power module. When provided in a power system with one or more other power modules and/or one or more other components, the switching semiconductor devices of a power module may form part of a power converter such as a half-bridge converter, a full-bridge converter, a buck converter, a boost converter, and the like. Power systems often deal with relatively high voltages and currents, and thus the switching semiconductor devices of a power module must similarly be capable of reliably switching these high voltages and currents.
Conventionally, the switching semiconductor devices of a power module have been silicon devices due to well-known processes for producing silicon switching semiconductor devices capable of reliably switching high voltages and currents. However, in recent years silicon carbide semiconductor switching devices for power modules have become popularized due to significant increases in switching speed and efficiency afforded by the use of silicon carbide. While power modules with silicon carbide semiconductor switching devices provide several performance benefits over their silicon counterparts, using silicon carbide semiconductor switching devices in a power module presents several challenges in the design thereof. Specifically, increased power density and concentrated electric fields in silicon carbide semiconductor switching devices often lead to issues with the long-term reliability of power modules incorporating them.
One way to measure the reliability of a power module is with a test known as a temperature, humidity, and bias (THB) test. Conventionally, THB tests have been conducted by providing a device under test (DUT) in an environment with fixed temperature (e.g., 85° C.) and relative humidity (e.g., 85%) while providing a fixed bias voltage (e.g., 100V) across the DUT in a reverse bias (i.e., blocking) state. The DUT should be capable of sustaining the bias voltage across the device without breaking down (i.e., exceeding a threshold for a leakage current through the device) for thousand(s) of hours to ensure the reliability of the DUT in a production environment. Recently, more rigorous THB tests for high power devices have emerged in which 80% of a rated voltage of the DUT is provided across the device during the same conditions discussed above. Known in the industry as a “THB80” or “HV-H3TRB” (high-voltage, high-temperature, high-reverse-bias) tests, these reliability tests more accurately reflect the reliability of a device in use, and are crucial indicators of reliability for power modules that will be operated in harsh environments such as outdoor power systems.
Notably, THB tests can be performed both at a die level and at a module level. When a THB test is performed at the die level, a semiconductor die is subjected to the conditions discussed above and the reliability of the die is assessed. When a THB test is performed at the module level, an assembled power module including several semiconductor die is subjected to the conditions discussed above and the reliability of the module as a whole is assessed. THB tests performed at the module level are significantly more difficult to pass, as the complexity of a power module is significantly greater than that of a single semiconductor die and thus presents many more points of failure when compared to a semiconductor die. Further, THB tests performed at the module level are much more likely to indicate the real-world reliability of a power module when compared to extrapolations of reliability from THB tests performed on the semiconductor die included within the power module. As power modules including silicon carbide switching semiconductor devices have become more popular, customer demand for these power modules for applications in harsh environments has similarly grown. However, the reliability of power modules including silicon carbide semiconductor switching components has thus far proved an impediment to their adoption in these spaces. Specifically, power modules including silicon carbide semiconductor switching components have thus far been incapable of satisfactory performance in the THB80 tests discussed above.
In light of the above, there is a present need for power modules including silicon carbide semiconductor switching components with improved ruggedness and reliability.